Fair : nf (word coming from low latin fair "Market, fair"). Large public market where various kinds of goods are sold and which takes place on fixed dates and places.
Show off: indulge in a life of debauchery (bombs, binge, Party, wot).
View: Motherfucker, Screw up, Fairground, Foolish.
The expression: "To get along like thieves": Very well to get along.
A thief being, according to Littré, "the one who commits a theft, who steals furtively", the expression get along like thieves (XNUMXth century version) was about accomplices who got along to prepare a bad move.
It was in the XNUMXth century that fair is added, those fairs which designate large public markets where all kinds of articles are presented and offered for sale. The kind of place where nasty things to the detriment of both merchants and visitors can be easily done.
In this expression, the notion of brigand associated with thief has gradually been lost, and, if it is easily used today to designate two kids who get along so well that we imagine that they could very well do stupid things together, it can also simply designate people who get along wonderfully, without any other connotation.